Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 24
Other IP–Related Protocols

The Internet Control Message Protocol (ICMP) is an extension of the IP layer. This is the reason that it uses an IP header and not a UDP (User Datagram Protocol) header. The purpose of ICMP is to report or test certain conditions on the network. IP delivers data and has no other form of communication. ICMP provides some error reporting mechanism for IP. Basically, it allows internet devices (hosts or routers) to transmit error or test messages. These error messages may be that a network destination cannot be reached or they may generate/reply to an echo request packet (PING, explained later).

The Internet Group Management Protocol (IGMP) is an extension of the IP protocol that allows for multicasting to exist for IP. The multicast address already existed for IP but there was not a control protocol to allow it to exist on a network. IGMP is a protocol that operates in workstations and routers and allows the routers to determine which multicast addresses exist on their segments. With this knowledge, routers can build multicast trees allowing multicast data to be received and propagated to their multicast workstations. IGMP headers are used as the basis for all multicast routing protocols for IPv4.

RSVP is called the resource reservation protocol and allows some semblance of Quality of Service (QoS) to exist using IP. It used to be we could increase the speed of a network to allow more bandwidth on which to fit hungry applications. With that capability, QoS was essentially ignored. However, bandwidth cannot continually expand. The Internet was not provisioned for Quality of Service, and RSVP is the first attempt to allow for it. Its benefits are apparent in multicasting applications, but it can be used with unicast applications as well. It allows stations on the network to reserve resources via the routers on the network.

ARP is not really part of the network layer; it resides between the IP and data–link layers. It is the protocol that translates between the 32–bit IP address and a 48–bit Local Area Network address. ARP is only used with IPv4; IPv6 has no concept of ARP. Since IP was not intended to run over a LAN, an address scheme was implemented to allow each host and network on the internet to identify itself. When TCP/IP was adapted to run over the LAN, the IP address had to be mapped to the 48–bit datalink or physical address that LANs use, and ARP is the protocol that accomplishes it.